Presence detecting device for automatic doors have been heretofore known and/or utilized wherein a plurality of infrared emitters and receivers are utilized to provide a detection area at the threshold of the automatically actuated door (see for example U.S. Pat. Nos. 4,823,010, 4,733,081, 4,697,383, 4,669,218, 4,565,029, and 4,698,937). In general, such devices have utilized single clock synchronization of infrared emitters to project a beam of energy in a detection area and receivers together with detector circuitry to receive reflected energy and send an operational signal when a target is detected at the threshold of a door. Such devices have, in some cases, employed a variety of means to prevent the door itself from being operationally treated as a target.
Various arrangements to compensate for variations in environmental factors in the detection area to minimize the number of malfunctions of the automatic door are also known and/or have been utilized. Such arrangements include manual threshold sensitivity adjustments (see for example U.S. Pat. No. 4,823,010) and/or utilization of elaborate and expensive multiple integration circuits (see for example U.S. Pat. No. 4,733,081, wherein the rate of change, or variation, of signal is utilized to discriminate a human body from the background signal received by a detector).
In addition, such devices and methods as have been heretofore known have been subject to false triggers, resulting in opening of a door where no body, such as a human body, shopping cart or the like, is present, or when transient and short lived occurrences are detected such as the reflection of sunlight from moving reflective surfaces or the reflection of the emitted infrared energy from a falling leaf or snowflake in the detection area.
It would thus be desirable to provide a simple and inexpensive presence detecting apparatus and method which includes automatic adjustment of the sensitivity of such an apparatus to compensate for changes in the environment in a detection area (such as snowfall, rain, daylight and darkness) while providing means for ignoring random and short lived transient occurrences. Such an apparatus would preferably be capable of asynchronous operation of the emitter and receiver/detector stages and resolve the problem of detecting movement of the door itself in the detection area simply and inexpensively. Further improvement in such heretofore known devices and methods could thus still be utilized.